I'm trying to install a set of Adventure Designs highway pegs on Tiger 800 OEM crash bars. I tightened them to the point that it appears I've put a small dent in bars but I can still spin them with just a small amount of pressure. I don't have the balls to tighten them any further.

In the past I've drilled a hole and put in a screw or even used Gorilla Glue. I really can't do that with this set.

Maybe you can line the U-bolt with a strip of metal to spread the clamping force over a larger surface area? Something in the neighborhood of 16ga, might be able to find something in the plumbing or elec. dept of your hardware store.
Too bad your crashbars don't have a short section of square tube to clamp to. Got any pictures?
Might also be able to make your own clamp blocks along the lines of handlebar mounts.

Take a look at some of these accessory pegs. The bolts have a pair of spacers with a conical edge. The conical spacers help tighten the wrap of the clamp on the tubing. If the tubing isn't the right size to make the little spacers work well, see if a few washers under the spacers will help?

Also, try wrapping the tube or lining the clamp with some self-adhering grit strips like one would use on wooden steps/treads to prevent slipping when they are wet? The strips are cheap and if you tear one up in a get-off, it is easy to add another wrap.

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"beware the grease mud. for therein lies the skid demon."-memory from an old Honda safety pamphlet

I'd ditch the U-bolt and fabricate up some SS strap in a saddle clamp shape, and use a couple of bolts instead, or if your material is thin enough you can have the clamp run under the U-bolt. The trick is to have it mechanically attached to your foot pegs. Originally I was thinking about welding pads to your u-bolts but I like this approach better.